Heterocyclic compounds part _IV (Pyrrole)

Orientation of Electrophilic Substitution on Pyrrole, Chemical Reactions of Pyrrole

1. Our Inspiration 1

2. A) Heterocyclic Compounds:

3. a)5-membered Heterocyclic compounds ( i ) Pyrrole or Azole : (Aza-cyclo-penta-2,4-diene) C 4 H 5 N

4. Explaination on the basis of Resonance stability of intermediate Carbonium ion (σ-complex) formed by the attack of electrophile (E + ): Orientation of Electrophilic Substitution in Pyrrole :

5. Attack of E + at ?

6. Orientation of Electrophilic Substitution in Pyrrole : [email protected] Prepared by Dr Pramod R Padole Attack of E + at 2-position (at C-2): 2 Attack of E + at 3-position (at C-3): 1 In pyrrole , electrophilic substitution occur at

7. Orientation of Electrophilic Substitution in Pyrrole : Q.1) Discuss / Describe the orientation of electrophilic substitution in pyrrole . (W-09, S-14 & W-18, 3- 4 Mark) Q.2) Why Electrophilic substitution in pyrrole takes place at two (2) and five (5) positions. (S-13 & W-14, 3 Mark) Q.3) On the basis of resonance structure of pyrrole , explain why it undergoes electrophilic substitution at 2-position and not at 3-position. (S-13, 4 Mark) Q.4) Why pyrrole undergoes electrophilic substitutions reactions preferably at C 2 and C 3 positions? (W-13, 3 Mark) Q.5) Explain the orientation of electrophilic substitution in case of pyrrole . (S-15, 4 Mark) Q.6) Substitution of an electrophile in pyrrole mainly occurs at position number: (S-17 & W-18, ½ Mark) (a) 1 (b) 2 (c) 3 (d) 4 Q.7) At what positions pyrrole undergoes electrophilic substitution? Give reasons. (S-17, 4 Mark)

8. Orientation of Electrophilic Substitution in Pyrrole : Resonating structures of an intermediate carbonium ion (σ-complex) formed by the attack of electrophile (E + ) at 2-position and 3-position are given below: (1) Attack of the electrophile at 3-position in pyrrole leads to an intermediate with only two resonance structures (less stable).

9. Orientation of Electrophilic Substitution in Pyrrole : ( 1) Attack of the electrophile at 3-position in pyrrole leads to an intermediate with only two resonance structures (less stable).

10. Orientation of Electrophilic Substitution in Pyrrole : ( 1) Attack of the electrophile at 3-position in pyrrole leads to an intermediate with only two resonance structures (less stable).

11. Orientation of Electrophilic Substitution in Pyrrole : ( 1) Attack of the electrophile at 3-position in pyrrole leads to an intermediate with only two resonance structures (less stable).

12. Orientation of Electrophilic Substitution in Pyrrole : Resonating structures of an intermediate carbonium ion (σ-complex) formed by the attack of electrophile (E + ) at 2-position and 3-position are given below: (1) Attack of the electrophile at 3-position in pyrrole leads to an intermediate with only two resonance structures (less stable).

13. Orientation of Electrophilic Substitution in Pyrrole : 2) Electrophilic Attack at 2-position (or 5-position ): Pyrrole undergoes electrophilic substitution at the 2-position. Attack of the electrophile at 2-position in pyrrole leads to an intermediate with three resonance structures. That is, the intermediates produced by attack at 2-position is more stable .

14. Orientation of Electrophilic Substitution in Pyrrole : . 2-electrophilic (or 5-electrophilic) attacked intermediate carbocation which is more stable due to more number of resonance structures and greater delocalization of positive charge than 3-electrophilic (or 4-electrophilic) attacked carbonium ion. That is the reason that, in pyrrole electrophilic attack occurs at 2-position (or 5-position) is more stable rather at 3-position (or 4-position).

15. Chemical Reactions of Pyrrole : Or Electrophilic Substitution Reactions: Pyrrole undergoes electrophilic substitution reactions at 2-position .

16. Chemical Reactions of Pyrrole : First Second Third Fourth Halogenation : Pyrrole undergoes halogenation rapidly due to activation of pyrrole ring. Nitration: Reaction with cold solution of HNO 3 in Acetic anhydride: Or Preparation of 2-nitro-pyrrole from Pyrrole : Sulphonation : Reaction with SO 3 in Pyridine: Friedel –Craft Acylation ( Acetylation ): or Reaction with Pyrrole & Acetic anhydride at 250 o C:

17. Chemical Reactions of Pyrrole : (a) Chlorination: (c) Iodination: Reaction with Iodine in presence of aqueous KI solution: 1) Halogenation : (b) Bromination : Reaction with Bromine in presence of ethanol at 0 o C: Reaction with Chlorine: Reaction with sulphuryl chloride (SO 2 Cl 2 ) in ether at 0 o C (273 K) :

18. Chemical Reactions of Pyrrole : (a) Chlorination: ( i ) Reaction with Chlorine: (ii) Reaction with sulphuryl chloride (SO 2 Cl 2 ) in ether at 0 o C (273 K) : When pyrrole is reacted or treated or heated with chlorine (5 equivalents of Cl 2 ); to form penta-chloro-pyrrole .

19. Chemical Reactions of Pyrrole : (a) Chlorination: (ii) Reaction with sulphuryl chloride (SO 2 Cl 2 ) in ether at 0 o C (273 K) : When pyrrole is reacted or treated or heated with sulphuryl chloride (SO 2 Cl 2 ) in ether at 0 o C (273 K) ; to form 2, 3, 4, 5 – tetra chloro-pyrrole . Q.1) Complete the following reaction. (W-18, 2 Mark) ( sulfuryl chloride, SO 2 Cl 2 , is a source of  chlorine  whereas thionyl chloride, SOCl 2 is a source of   chloride  ions)

20. Chemical Reactions of Pyrrole : (b) Bromination : Reaction with Bromine in presence of ethanol at 0 o C: Q.1) How will you convert: Pyrrole to tetra- bromo pyrrole ? (W-13, 2 Mark) When pyrrole is reacted or treated with bromine (4 equivalents) in presence of ethanol at 0 o C (or 273K); to form 2,3,4,5 –tetra bromo-pyrrole .

21. Chemical Reactions of Pyrrole : (c) Iodination: Reaction with Iodine in presence of aqueous KI solution: Q.1) How will you obtain: 2,3,4,5 – tetra- iodo - pyrrole from pyrrole ? (S-16, 2 Mark) When pyrrole is reacted or treated with iodine (4 moles) in presence of in aqueous potassium iodide (KI) solution; to form 2,3,4,5 – tetra- iodo - pyrrole (or iodole) .

22. Prepared by Dr Pramod R Padole Reaction with cold solution of HNO 3 in Acetic anhydride: Or Preparation of 2-nitro-pyrrole from Pyrrole : (2) Nitration :

23. Reaction with cold solution of HNO 3 in Acetic anhydride: When pyrrole is reacted or treated with cold solution of nitric acid (HNO 3 ) in acetic anhydride at about 0-5 o C; to form 2- nitro- pyrrole Q.1) Complete the following reaction. (S-10, S-12 & W-14, 2 Mark) Q.2) What happen when pyrrole reacts with Nitric acid? (W-12, 2 Mark)

24. Reaction with cold solution of HNO 3 in Acetic anhydride: When pyrrole is reacted or treated with cold solution of nitric acid (HNO 3 ) in acetic anhydride at about 0-5 o C; to form 2- nitro- pyrrole

25. Sulphonation : Reaction with SO 3 in Pyridine: Q.1) What happen when, Pyrrole is heated with sulphur trioxide in pyridine ? ( S-10, 2 Mark) Q.2) Complete the chemical equation or reaction. (C 2 H 4 Cl 2 ) ( S-11, W-13, W-16 & W-17, 2 Mark ) Q.3) What is the action of sulphur trioxide in pyridine on pyrrole ? ( W-12, 2 Mark) Q.4) How will you convert: Pyrrole to pyrrole-2-sulphonic acid. ( W-13 & W-14, 2 Mark) Q.5) Complete the following reaction. ( W-13, 2 Mark)

26. Sulphonation : Reaction with SO 3 in Pyridine: When pyrrole is heated with sulphur trioxide (SO 3 ) in pyridine, as a solvent, at about 90-100 o C (363-373 K); to from pyrrole – 2-sulphonic acid (or 2-pyrrole-sulphonic acid ).

27. Friedel –Craft Acylation ( Acetylation ): or Reaction with Pyrrole & Acetic anhydride at 250 o C: Q.1) What happen when: Pyrrole is heated with acetic anhydride at 523 K ? ( W-09 & S-15, 2 Mark) Q.2) Complete the following reaction: ( S-18, 2 Mark )

28. Reaction with Pyrrole & Acetic anhydride at 250 o C: When pyrrole is heated with acetic anhydride at 250 o C (523K), pyrrole undergoes acylation ; to form 2-acetyl pyrrole . No catalyst is required in this reaction. Note: Pyrrole does not give acetylation reaction with acetyl chloride; because pyrrole contain imino hydrogen (>NH).

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